Merging text files based on their filename

Question

I have hundred of files with the following format:

2013_01_19_xyxyxyx.txt
2013_01_19_xyxyxyx.txt
2014_03_21_xyxyxyx.txt
2014_03_21_zxzxzxz.txt
2014_03_22_abababa.txt
2014_03_22_cbcbcbc.txt
etc.

I would like to merge files into:

2013_01_19.txt
2014_03_21.txt
2014_03_22.txt

How could I achieve this ?

Answer 1

The following should work provided you don't have any underscores in the last part of any of the filenames (and nothing else in the directory that matches the glob):

for file in ????_??_??_*.txt; do
  echo cat "$file" ">>""${file%_*}.txt"
done

Remove the echo part and the quotes around the >> when you are sure you have what you want.